The present invention relates to a platinum/ruthenium alloy catalyst containing finely dispersed alloy particles on a powdery, electrically conductive carrier material. The catalyst is particularly suitable as an anode catalyst for fuel cells having a polymer electrolyte membrane.
Fuel cells are, in principle, gas-operated batteries in which the energy derived from the reaction of hydrogen and oxygen is directly converted into electrical energy. The instant invention describes the preparation of catalysts for fuel cells, in particular the preparation of supported catalysts based on platinum and platinum alloys for PEM fuel cells (PEM=polymer electrolyte membrane). This type of fuel cell is gaining growing importance as a source of current for motor vehicles driven by electric motors because of its high energy density and robustness.
Compared to conventional combustion engines, fuel cells display very low emissions with, at the same time, very high efficiency. When hydrogen is used as the fuel gas, water is the only emission formed on the cathode side of the cell. Motor vehicles with this type of drive are termed ZEV (Zero Emission Vehicles).
Hydrogen is, however, too expensive at the present time and causes problems in storage and in the fuelling of vehicles. This explains the growing importance of the alternative of generating hydrogen directly on board the vehicle through the reforming of methanol. The methanol stored in the vehicle tank is converted in a steam reforming process at 200-300.degree. C. into a hydrogen-rich fuel gas with carbon dioxide and carbon monoxide as secondary constituents. After converting the carbon monoxide by the shift reaction, preferential oxidation (PROX) or other purification process, this fuel gas is conducted directly to the anode side of the PEM fuel cell. The reformed gas theoretically consists of 75 vol. % hydrogen and 25 vol. % carbon dioxide. In practice, however, this gas still contains nitrogen, oxygen and, depending on degree of purity, fluctuating amounts of carbon monoxide (up to 1 vol. %).
Catalysts based on platinum and platinum alloys are used as catalysts on the anode side and on the cathode side of the PEM fuel cell. These consist of finely divided precious metal particles that are precipitated onto a conductive carrier material (generally carbon black or graphite). The precious metal content ranges from 10 to 50 wt. %, based on the total weight of the catalyst.
Since conventional platinum catalysts are very sensitive to carbon monoxide poisoning, the carbon monoxide content of the fuel gas must be lowered to below 10 ppm to prevent performance loss in the fuel cells due to poisoning of the anode catalyst. This applies in particular to the PEM fuel cell that is particularly sensitive to carbon monoxide poisoning with its low working temperatures of 70 to 100.degree. C.
The instant invention relates to the preparation of supported catalysts on the basis of bimetallic platinum/ruthenium alloy catalysts that display a high resistance to carbon monoxide poisoning. Carbon monoxide contents of the reformed gas in excess of 100 ppm should be possible and lead to virtually no perceptible performance losses in the PEM fuel cell.
The use of novel catalysts of this kind on the anode side of the PEM fuel cell can reduce the number of process steps needed to remove carbon monoxide from the fuel gas. This leads to a substantial reduction in the system costs, to an improvement in the efficiency of the system and makes the entire system smaller. The new catalysts are therefore of great importance for the introduction of the PEM fuel cell in motor vehicles.
The problem of the poisoning of platinum catalysts by carbon monoxide has been known for a long time. In view of its special molecular structure, carbon monoxide is adsorbed onto the surface of the platinum, thereby blocking access for the hydrogen molecules of the fuel gas to the catalytically active centers of the platinum.
By adding water, the adsorbed carbon monoxide can be oxidized to carbon dioxide and can then be removed from the surface of the catalyst. It is also known that the tolerance of the platinum catalyst to carbon monoxide poisoning can be improved by alloying or doping the platinum with other metals.
EP 0 501 930 B1 describes for example quaternary alloys of platinum, nickel, cobalt and manganese as anode catalyst of phosphoric acid fuel cells (PAFC: phosphoric acid fuel cell) that possesses good resistance to carbon monoxide at the high operating temperatures of a phosphoric acid fuel cell of 160 to 200.degree. C. The size of the alloy particles is in the region of 3 nm. At the high operating temperatures of the phosphoric acid fuel cell, however, there is at the outset a reduced tendency for the carbon monoxide to adsorb onto metal surfaces than at the low operating temperatures of a PEM fuel cell.
L. W. Niedrach et.al. (J. Electrochemical Techn. 5, 1967, S.318) describe the use of Pt/Ru catalysts as CO-tolerant anode catalysts for sulphuric acid fuel cells. These materials consist of fine Pt/Ru alloy powders with high specific surfaces. They are prepared using the so-called ADAMS process in a melt of platinum chloride, ruthenium and sodium nitrate at 500.degree. C. Because of the high temperatures during preparation, these catalysts are present as Pt/Ru alloys. The materials are not fixed to a carrier and therefore do not constitute supported catalysts. Moreover there is no information on their use in a PEM fuel cell.
EP 0 549 543 B1 describes a process for the preparation of supported catalysts that contain highly dispersed metal particles with mean particle sizes of under 2 nm. The process consists in reducing metal ions in a suspension of the carrier material by means of a reducing agent in the presence of carbon monoxide and simultaneously precipitating them onto the carrier. The carbon monoxide present is adsorbed onto the precipitating metal particles and thereby hinder further particle growth. Following completed precipitation, the catalyst is washed and dried at a temperature below 100.degree. C. in a reducing atmosphere. The carbon monoxide is thereby desorbed. Example 4 describes the preparation of a platinum/ruthenium catalyst on carbon with a mean particle size of the platinum/ruthenium particles of 1.7 nm. In this case, however, the catalyst is not an alloy catalyst, since the adsorption of the carbon monoxide on the metal particles during precipitation prevents the formation of an alloy. Nor is any alloy formed during the subsequent temperature treatment up to 100.degree. C. No statement is made regarding the properties of this catalyst in use as anode catalyst in a PEM fuel cell with a reformed gas containing carbon monoxide.
A platinum/ruthenium alloy catalyst on a carrier material has been commercially available for some time. This is a Pt/Ru alloy catalyst with a precious metal loading between 5 and 40 wt. % and a Pt/Ru atomic ratio of 1:1. This catalyst displays a uniform alloy phase that can be determined using XRD. Examinations of this catalyst indicated an unsatisfactory tolerance to carbon monoxide, in particular at carbon monoxide concentrations over 100 ppm and residual oxygen in the fuel gas.
In a recent paper, M. Iwase and S. Kawatsu report on the development of CO-tolerant anode catalysts (M. Iwase and S. Kawatsu, Electrochemical Society Proceedings, Volume 95-23, S. 12). In this paper, the best results were achieved with a Pt/Ru alloy catalyst in which the formation of the alloy was obtained by a special temperature treatment. The voltage drop at a current density of 0.4 A/cm.sup.2 was nonetheless still ca. 200 mV at a CO-content of 100 ppm. This is still too high for practical use. Still poorer results were, however, achieved with an unalloyed Pt/Ru catalyst.
The positive effect of the ruthenium on the resistance to poisoning is attributed to the fact that, in the presence of oxygen when ruthenium is present, carbon monoxide is oxidized to carbon dioxide which displays a lesser tendency to adsorption on metal surfaces than does carbon monoxide.
It is therefore an object of the instant invention to prepare a platinum/ruthenium alloy catalyst on a carrier that displays improved tolerance to carbon monoxide.
A further object of the instant invention is to provide a catalyst suitable for operation with carbon monoxide-, nitrogen- and oxygen-containing fuel gases which also displays a voltage drop that is as low as possible with high current densities at carbon monoxide contents of the fuel gas of more than 100 ppm.